I have some questions in regards to "dyanmic reset limits" and "Use PV for BKCAL_OUT"
To make things clear I will use a real example in our plant.
10-FIC-140 (slave) receives a flow cascade set point from 10-LIC-139 (master). To use the dynamic reset limits, please let me know if this is correct:
I would enable "dynamic reset limit" on 10-LIC-139 (master), and "use PV for BKCAL_OUT" on 10-FIC-140 (slave). The one question I have is that with the current configuration, someone put a scaler block on the BKCAL_OUT of 10-FIC-140, to scale it from its flow range to 0-100%, which is then fed into the BKCAL_IN of 10-LIC-139. Was this done because BKCAL_IN is a 0-100% scale?(it is in EU of out scale, which is typically 0-100%) How will the BKCAL_IN see the PV of the slave controller?
Also, how exactly does the dynamic reset limit work?
Thank you in advance!
Jayme
This might not be the best explanation but it should help with some of the understanding : 1drv.ms/.../s!ApBGsgbP06JAgsgSXTZxGr8JmXgBDA
First, thanks to Paul Hughes for sharing that interesting paper!
Jayme,
Regarding your question about the "Flow Range to 0-100%" scalar in the path of the 10-FIC140/BKCAL_OUT path to the 10-LIC-139/BKCAL_IN. Apparently a decision was made to keep the OUT_SCALE on the primary (aka master) controller 10-LIC-139 as 0-100% (sometimes to match the previous DCS implementation) rather than the PV_SCALE of the secondary (aka slave) loop, 10-FIC-140. You should also find a "0-100% to Flow Range" scaler in the path from 10-LIC-139/OUT to 10-FIC-140/CAS_IN to convert the 0-100% OUT_SCALE of the LIC to the PV_SCALE of the FIC.
When you choose "Use PV for BKCAL_OUT" on the secondary loop,10-FIC-140, when in the AUTO, CAS or RCAS mode, the BKCAL_OUT will be the PV of the lsecondary loop (rather than the SP if this option is not chosen). The PV, in EU's of the flow loop, will go to the Scalar you mentioned and be converted to 0-100% which is the OUT_SCALE of master loop10-LIC-139. Note that you could make the OUT_SCALE of the Primary loop be the same as the PV_SCALE of the secondary loop and these Scalar blocks would not be needed.
Now, let's talk about how DeltaV specifically implements Dynamic Reset Limiting (also known as external reset feedback). DeltaV uses a "positive feedback filter", with the time constant equal to the Reset time, to produce the integral action. Referring to the attached Figure for BOL showing how the DeltaV PID is implemented, without DRL, the OUT of the PID, after the OUT limits are applied, provides the input to the positive feedback filter to produce the integral action. A any contribution to the OUT from the P or D component of the PID OUT will induce integral action if the OUT is not limited. In other words, if the OUT hits a limit, the input to the feedback filter does not move and no integral action is created. As you see in the diagram, when DRL is selected, the input to the positive feedback is the BKCAL_IN. So, if the BKCAL_IN stops moving, the integral action will stop. Or, if the BKCAL_IN moves "slower than normal" the integral action will be effectively slowed down.
When the secondary loop has "Use PV for BKCAL_OUT", the PV will be used as the BKCAL_IN of the Primary loop. Normally the PV of a loop will closely follow the SP and so DRL doesn't have much impact in this regard. However, the secondary loop's response time slows down for some reason (change in process dynamics/gain) DRL will slow down the effective integral action of the Primary loop and maintain stability of the cascade loop. DeltaV uses the limited status (SP or OUT) of the downstream secondary loop to stop the integral action of the primary upstream loop and avoid integral windup. However, DRL COULD stop integral action of the primary BEFORE the secondary becomes SP or OUT limited if the PV quits moving. So, DRL provides some protection of stability of cascade loops and possible soon stopping of integral windup.
There is also another use of DRL that is related to "pre-loading" the OUT of a primary controller such as in batch processes. Also, a dead time function can be placed in the external feedback path to accommodate the control of dead time dominant processes! This Control Global article by Greg Shinskey discusses both aspects and more! https://www.controlglobal.com/articles/2006/095/
Hope this helps! Let me know if more clarification is needed!
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